Nonvolatile memory devices

ABSTRACT

Nonvolatile memory devices and methods for fabricating the same are provided. The device includes first and second base patterns disposed under floating and selection gates, respectively, at an active region. A channel region is formed in the active region between the first and second base patterns, and source and drain regions are formed in the active region adjacent to the first and second base patterns, respectively. The method includes forming first and second base patterns on a semiconductor substrate to be separated from each other by a predetermined space. A channel region is formed in the semiconductor substrate between the first and second base patterns. Source and drain regions are formed in the semiconductor substrate adjacent to the reverse side of the channel region on the basis of the first and second base patterns, respectively. A tunnel oxide layer is formed on a predetermined region of the channel region. A memory gate is formed to cover the first base pattern and the tunnel oxide layer. A selection gate pattern is then formed to cover the second base pattern.

RELATED APPLICATION

[0001] This application is a divisional of U.S. application Ser. No. 10/334,952, filed on Dec. 31, 2002, which relies for priority upon Korean Patent Application No. 02-00451, filed on Jan. 4, 2002, the contents of which are herein incorporated by reference in their entirety.

FIELD OF THE INVENTION

[0002] The present invention relates to nonvolatile memory devices and methods for fabricating the same. More specifically, the present invention is directed to electrically erasable programmable read-only memories (EEPROMs) of a floating gate tunnel oxide (FLOTOX) type in which memory transistors and selection transistors are connected in series, and methods for fabricating the same.

BACKGROUND OF THE INVENTION

[0003] Like erasable programmable read only memory (EPROM) cells, EEPROM cells include floating gates and store data by injecting electrons into the floating gate or erase data by emitting the electrons from the floating gate. However, EEPROMs adopt a manner of injecting or emitting electrons, which is quite different from EPROMs.

[0004] In EPROMs, electrons are injected into a floating gate by hot carrier injection (HCI) and emitted from the floating gate by energy of ultraviolet rays irradiated to the floating gate. By comparison, EEPROMs employ tunneling induced by a thin tunnel insulation layer for injecting and emitting electrons. That is, if a strong electric field of about 10 MeV/cm is applied to both surfaces of the tunnel insulation layer, a current flows through the tunnel insulation layer, which is normally called “Fowler-Nordheim (F-N) tunneling”. In the EEPROMs, the F-N tunneling is used for injecting and emitting electrons.

[0005] In the case of the EEPROMs such as FLOTOX-type memories, a single memory cell includes a pair of transistors, which are connected in a series. One is a selection transistor for selecting cells, and the other is a memory transistor for storing data. The memory transistor includes a floating gate for storing electric charge, a control gate electrode for controlling a memory transistor, and a gate interlayer dielectric layer interposed therebetween.

[0006]FIGS. 1 through 3 are cross-sectional views for illustrating a method of fabricating a conventional nonvolatile memory device.

[0007] Referring to FIG. 1, a gate insulation layer 102 is formed on a semiconductor substrate 100, and a channel diffusion layer 110 is formed in a predetermined region of the semiconductor substrate 100. Although not shown in the drawings, before forming the gate insulation layer 102, a device isolation layer is formed at a predetermined region of the semiconductor substrate to define an active region. The gate insulation layer 102 is formed on the active region. A portion of the gate insulation layer 102 is removed to expose a predetermined region of the channel diffusion layer 110. A tunnel insulation layer 118 is formed on the exposed region. That is, the tunnel insulation layer 118 is disposed on the channel diffusion layer 110. A lower conductive layer and a dielectric layer are sequentially formed on an entire surface of the semiconductor substrate 100. Thereafter, the lower conductive layer and the dielectric layer are successively patterned to form a floating gate pattern 120 a covering the channel diffusion layer 110, and a lower selection gate pattern 120 b separated from the floating gate pattern 120 a by a predetermined space. A gate interlayer dielectric layer 122 a is formed on the floating gate pattern 120 a, and an interlayer dielectric pattern 122 b is formed on the lower selection gate pattern 120 b.

[0008] Referring to FIG. 2, a thermal process is performed on the resultant structure to form a sidewall insulation layer 124 on sidewalls of the floating gate pattern 120 a and the lower selection gate pattern 120 b. An upper conductive layer 126 is formed on an entire surface of the resultant structure where the sidewall insulation layer 124 is formed.

[0009] Referring to FIG. 3, the upper conductive layer 126 is patterned to form a control gate electrode 126 a on the gate interlayer dielectric layer 122 a and at the same time form an upper selection gate 126 b on the interlayer dielectric pattern 122 b.

[0010] Although not shown in the drawings, in another method, after forming the lower conductive layer, the upper conductive layer, the dielectric layer, and the lower conductive layer may be successively patterned to form a control gate electrode as well as a gate interlayer dielectric pattern and a floating gate pattern, which are self-aligned to the control gate electrode. At the same time, an upper selection gate as well as an interlayer dielectric pattern and a lower selection gate pattern, which are self-aligned to the upper selection gate, may be formed.

[0011] The floating gate pattern 120 a, the gate interlayer dielectric layer 122 a, and the control gate electrode 126 a constitute a gate pattern of the memory transistor. The lower selection gate pattern 120 b, the interlayer dielectric pattern 122 b, and the upper selection gate 126 b constitute a gate pattern of the selection transistor.

[0012] Next, impurities are injected into the semiconductor substrate between the memory gate pattern and the selection gate pattern to form a channel region 110 a including the channel diffusion layer 110. A source region 108 is formed in the semiconductor substrate adjacent to the memory gate pattern. A drain region 112 is formed in the semiconductor substrate adjacent to the selection gate pattern.

[0013]FIG. 4 is a cross-sectional view for illustrating disadvantages of the conventional nonvolatile memory device. Referring to FIG. 4, a cell array of the nonvolatile memory device of FLOTOX type includes memory cells, each of which has a selection transistor and a memory transistor. In the foregoing cell array, adjacent memory cells are disposed symmetrically with respect to each other. That is, each memory cell has a source region in common with a memory cell adjacent to one side thereof, and has a drain region in common with a memory cell adjacent to the other side thereof. Accordingly, when the floating gate pattern 120 a and the lower selection gate pattern 120 b are formed to be misaligned, spaces between the source regions 108 and the channel regions 110 a become irregular. This may cause dispersion of threshold voltages of memory transistors to be increased.

SUMMARY OF THE INVENTION

[0014] It is therefore an object of the present invention to provide nonvolatile memory devices, of which memory cells include channel regions and source regions that have regular spaces therebetween, and methods of fabricating the same.

[0015] It is another object of the present invention to provide nonvolatile memory devices, of which cell arrays include memory transistors having threshold voltages of low dispersion, and methods of fabricating the same.

[0016] In accordance with objects of the present invention, provided is a nonvolatile memory device, which in one embodiment is of FLOTOX type, and in which a memory transistor and a selection transistor are connected in a series. The device comprises first and second base patterns, which are disposed on a semiconductor substrate to be separated from each other by a predetermined space, and a channel region formed in the semiconductor substrate between the first and second base patterns. Source and drain regions, which are separated from the channel region by the first and second base patterns, are disposed in the semiconductor substrate adjacent to the first and second base patterns, respectively. A memory gate covers the first base pattern and is extended to a predetermined region of the channel region. A tunnel insulation layer is interposed between the memory gate and the channel region. The selection gate covers the second base pattern.

[0017] In a first embodiment of the present invention, the memory gate may include a floating gate pattern covering the first base pattern and a predetermined region of the channel region, and a control gate electrode covering the floating gate pattern. A gate interlayer dielectric layer is interposed between the control gate electrode and the floating gate pattern. In addition, the selection gate may comprise a lower selection gate pattern, an interlayer dielectric layer and an upper selection gate, which are sequentially stacked on the second base pattern. In this case, the upper selection gate and the lower selection gate pattern are connected to each other, penetrating the interlayer dielectric layer.

[0018] The method of fabricating the nonvolatile memory device comprises forming first and second base patterns, which are separated from each other by a predetermined space on a semiconductor substrate. A channel region is formed in the semiconductor substrate between the first and second base patterns. Source and drain regions, which are separated from the channel region by the first and second base patterns, are formed in the active regions adjacent to the first and second base patterns, respectively. A tunnel oxide layer is formed on a predetermined region of the channel region, and a memory gate is formed to cover the first base pattern and the tunnel oxide layer. In addition, a selection gate pattern is formed to cover the second base pattern. The memory gate includes a floating gate pattern, a gate interlayer dielectric layer, and a control gate electrode, which are sequentially stacked. The floating gate pattern covers the first base pattern and the tunnel oxide layer, and the control gate electrode is disposed on the floating gate. The gate interlayer dielectric layer is interposed between the control gate electrode and the floating gate pattern.

BRIEF DESCRIPTION OF THE DRAWINGS

[0019] The foregoing and other objects, features and advantages of the invention will be apparent from the more particular description of a preferred embodiment of the invention, as illustrated in the accompanying drawings in which like reference characters refer to the same parts throughout the different views. The drawings are not necessarily to scale, emphasis instead being placed upon illustrating the principles of the invention.

[0020]FIGS. 1 through 3 are cross-sectional views for illustrating a method of fabricating a conventional nonvolatile memory device.

[0021]FIG. 4 is a cross-sectional view for illustrating disadvantages of the conventional nonvolatile memory device.

[0022]FIG. 5 is a top plan view of a nonvolatile memory device according to a first preferred embodiment of the present invention.

[0023]FIG. 6 is a perspective view of the nonvolatile memory device according to the first preferred embodiment of the present invention.

[0024]FIGS. 7a, 8 a, 9 a, 10 a, 11 a, 12 a, 13 a, 14, and 15 are cross-sectional views for illustrating a method of fabricating the nonvolatile memory device according to the first preferred embodiment of the present invention, the cross-sectional views taken along a line I-I′ of FIG. 5.

[0025]FIGS. 7b, 8 b, 9 b, 10 b, 11 b, 12 b and 13 b are cross-sectional views for illustrating a method of fabricating the nonvolatile memory device according to the first preferred embodiment of the present invention, the cross-sectional views taken along a line II-II′ of FIG. 5.

[0026]FIG. 12c is a top plan view of the nonvolatile memory device as illustrated in FIGS. 12a and 12 b according to the first preferred embodiment of the present invention.

[0027]FIG. 16 is a top plan view of the nonvolatile memory device according to a second preferred embodiment of the present invention.

[0028]FIG. 17 is a perspective view of the nonvolatile memory device according to the second preferred embodiment of the present invention.

[0029]FIGS. 18a, 19 a, and 20 are cross-sectional views for illustrating a method of fabricating the nonvolatile memory device according to the second preferred embodiment of the present invention, the cross-sectional views taken along a line III-III′ of FIG. 16.

[0030]FIGS. 18b and 19 b are cross-sectional views for illustrating a method of fabricating the nonvolatile memory device according to the second preferred embodiment of the present invention, the cross-sectional views taken along a line IV-IV′ of FIG. 16.

[0031]FIG. 18c is a top plan view of the nonvolatile memory device as illustrated in FIGS. 18a and 18 b according to the second preferred embodiment of the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0032] The present invention will now be described more fully hereinafter with reference to the accompanying drawings, in which preferred embodiments of the invention are shown. The invention may, however, be embodied in different forms and should not be construed as limited to the embodiments set forth herein. Rather, these embodiments are provided so that this disclosure will be thorough and complete, and will fully convey the scope of the invention to those skilled in the art. In the drawings, the thickness of layers and regions are exaggerated for clarity. It will also be understood that when a layer is referred to as being “on” another layer or substrate, it can be directly on the other layer or substrate, or intervening layers may also be present.

[0033]FIGS. 5 and 6 are a top plan view and a perspective view, respectively, of a nonvolatile memory device according to a first preferred embodiment of the present invention. Referring to FIGS. 5 and 6, the present invention includes a device isolation layer disposed at a predetermined region of a semiconductor substrate to define an active region. A memory gate pattern and a selection transistor pattern cross in a row over the active region. The memory gate includes a control gate electrode 226 a crossing over the active region, and a floating gate pattern 220 a interposed between the control gate electrode 226 a and the active region. The memory gate further includes a gate interlayer dielectric layer 222 a between the floating gate pattern 220 a and the control gate pattern 226 a. A first base pattern 204 a is interposed between the floating gate pattern 220 a and the active region.

[0034] The selection gate pattern includes a lower selection gate pattern 204 b, an interlayer dielectric pattern 222 b, and an upper selection gate 220 b, which are sequentially stacked and cross over the active region. A second base pattern 204 b is disposed under the lower selection gate pattern 204 b to cross over the active region in parallel with the lower selection gate pattern 204 b. A tunnel insulation layer 218 exists on the active region between the first and second base patterns 204 a and 204 b. The floating gate pattern 220 a is extended from a top of the first base pattern 204 a to cover the tunnel insulation layer 218. A gate insulation layer 202, which is thicker than the tunnel insulation layer 218, exists on the active region surrounding the tunnel insulation layer 218. The gate insulation layer 202 may be interposed between the first base pattern 204 a and the active region, between the second base pattern 204 b and the active region, between the floating gate pattern 220 a and the active region, and between the lower selection gate pattern 220 b and the active region. A channel region is disposed in the active region between the first and second base patterns 204 a and 204 b. A source region 208, which is separated from the channel region 210 by the first base pattern 204 a, is disposed in the active region adjacent to the first base pattern 204 a, while a drain region 212, which is separated from the channel region by the second base pattern 204 b, is disposed in the active region adjacent to the second base pattern 204 b.

[0035] In the first preferred embodiment, in a cross-sectional view of the memory gate, a width of the floating gate pattern 220 a is larger than that of the control gate electrode 226 a. In a cross-sectional view of the selection gate pattern, a width of the lower selection gate pattern 220 b is larger than that of the upper selection gate pattern 226 b. The source region 208 crosses the device isolation layer Fox to be connected to adjacent source regions. Preferably, the device isolation layer Fox between the adjacent source regions is cut so that the active regions are connected, and the source region 208 can be disposed in the connected active regions. The floating gate pattern 220 a, the gate interlayer dielectric layer 222 a and the control gate electrode 226 a constitute a memory gate. The upper selection gate pattern 220 b, the interlayer dielectric pattern 222 b, and the upper selection gate pattern 226 b constitute a selection gate.

[0036]FIGS. 7a, 8 a, 9 a, 10 a, 11 a, 12 a, 13 a and 14 are cross-sectional views for illustrating a method of fabricating a nonvolatile memory device according to the first embodiment of the present invention, the cross-sectional views taken along a line I-I′ of FIG. 5. FIG. 15 is a cross-sectional view for illustrating a transformed example of the first preferred embodiment of the present invention, the cross-sectional view taken along a line I-I′ of FIG. 5.

[0037]FIGS. 7b, 8 b, 9 b, 10 b, 11 b, 12 b and 13 b are cross-sectional views for illustrating a method of fabricating a nonvolatile memory device according to the first embodiment of the present invention, the cross-sectional views taken along a line II-II′ of FIG. 5.

[0038] Referring to FIGS. 7a and 7 b, a device isolation layer Fox is formed at a predetermined region of a semiconductor substrate 200 to define active regions. A gate insulation layer 202, a base conductive layer 204, and an upper insulation layer 206 are sequentially formed on an entire surface of the resultant structure where the device isolation layer Fox is formed. The base conductive layer 204 is preferably composed of polysilicon. The upper insulation layer 206 is preferably an insulation layer having an etch selectivity with respect to the base conductive layer 204 and the gate insulation layer 202, for example, a silicon nitride layer.

[0039] Referring to FIGS. 8a and 8 b, the upper insulation layer 206 and the base conductive layer 204 are successively patterned to form first and second base patterns 204 a and 204 b. The first base pattern 204 a covers the active regions and the second base pattern 204 b is separated from the first base patterns 204 a to cross the active region. An upper insulation pattern 206 a covers the first and second base patterns 204 a and 204 b. The first base patterns 204 a cover the active region, and edges of the first base patterns 204 a are extended to overlap with the device isolation layer Fox. Groups including the first and second base patterns 204 a and 204 b are adjacent to each other in a memory cell array and disposed symmetrically. A channel region 210 is formed in the active region between the first and second base patterns 204 a and 204 b. A source region 208 is formed in the active region separated from the channel region 210 by the first base pattern 204 a. A drain region 212 is formed in the active region separated from the channel region 210 by the second base pattern 204 b. The source region 208 is adjacent to the first base pattern 204 a, while the drain region 212 is adjacent to the second base pattern 204 b. The channel region 210, the source region 208, and the drain region 212 may be formed in the active region by an ion implantation process using the first and second base patterns 204 a and 204 b and the upper insulation pattern 206 a as an ion implantation mask.

[0040] It is preferable that the adjacent source regions 208, which are separated by the device isolation layer Fox, are electrically connected to each other. For this, it is preferable not to form the device isolation layer Fox between the regions where the source regions 208 will be formed. Accordingly, the adjacent source regions 208 are connected to each other to form a common source line crossing the device isolation layer FOX.

[0041] Referring to FIGS. 9a and 9 b, a first sidewall spacer 214 a is formed on each of sidewalls of the first base patterns 204 a and the upper insulation patterns 206 a, which are sequentially stacked. A second sidewall spacer 214 b is formed on each of sidewalls of the second base patterns 204 b and the upper insulation patterns 206 a, which are sequentially stacked. The first and second sidewall spacers 214 a and 214 b are preferably insulation layers having an etch selectivity with respect to the gate insulation layer 202.

[0042] Referring to FIGS. 10a and 10 b, a photoresist pattern 216 is formed on the resultant structure where the first and second sidewall spacers 214 a and 214 b are formed. The photoresist pattern 216 exposes a predetermined portion of the gate insulation layer 202 between the first and second sidewall spacers 214 a and 214 b. By using the photoresist pattern 216 as an etch mask, the predetermined portion of the gate insulation layer 202 is etched to expose a portion of the semiconductor substrate 200.

[0043] The photoresist pattern 216 preferably exposes the first sidewall spacers 214 a and the gate insulation layer 202 between the first and second base pattern 204 a and 204 b. Thus, the edges of the exposed semiconductor substrate 200 are aligned to edges of the first sidewall spacer 214 a.

[0044] Referring to FIGS. 11a and 11 b, the photoresist pattern 216, the upper insulation pattern 206 a, and the first and second sidewall spacers 214 a and 214 b are removed. Continuously, a tunnel insulation layer 218, which is thinner than the gate insulation layer 202, is formed on the exposed semiconductor substrate. For instance, annealing can be performed into the semiconductor substrate to form a thermal oxide layer on the exposed semiconductor substrate, thereby enabling formation of the tunnel insulation layer 218. A lower conductive layer 220 and a dielectric layer 222 are conformally formed on an entire surface of the resultant structure where the tunnel insulation layer 218 is formed. The lower conductive layer 220 may be a polysilicon layer and the dielectric layer 222 may be an oxide-nitride-oxide (ONO) layer.

[0045] Referring to FIGS. 12a and 12 b, the dielectric layer 222 and the lower conductive layer 220 are successively patterned to form a floating gate pattern 220 a and a gate interlayer dielectric layer 222 a. The floating gate pattern 220 a covers the first base pattern 204 a and the tunnel insulation layer 218, and the gate interlayer dielectric layer 222 a covers the floating gate pattern 220 a. A lower selection gate pattern 220 b is then formed on the second base pattern 204 b to cross the active regions, and an interlayer dielectric pattern 222 b is formed on the lower selection gate pattern 220 b. The first base pattern 204 a and the floating gate pattern 220 a constitute a floating gate of a memory transistor. Likewise, the second base pattern 204 b and the lower selection gate pattern 220 b constitute a lower selection gate of the selection transistor. At this time, a sidewall of the first base pattern 204 a adjacent to the channel region 210 is preferably covered with the floating gate pattern 220 a. Similarly, a sidewall of the second base pattern 204 b adjacent to the channel region 210 is preferably covered with the lower selection gate pattern 220 b. This makes it possible to prevent edges of the first and second base patterns 204 a and 204 b from being etched, when misalignment arises during a photolithography process for forming the floating gate pattern 220 a and the lower selection gate pattern 220 b.

[0046] Next, a sidewall insulation layer 224 is formed on sidewalls of the floating gate and the lower selection gate. The sidewall insulation layer 224 is preferably formed on sidewalls of the floating gate pattern 220 a and the lower selection gate pattern 220 b. However, in the event that the sidewall of the first base pattern 204 a or the sidewall of the second base pattern 204 b is exposed, the sidewall insulation layer 224 may cover the exposed sidewall of the first or second base pattern 204 a or 204 b. That is, every structure having conductivity, which is formed on the semiconductor substrate, is surrounded by an insulation layer. The sidewall insulation layer 224 may be a thermal oxide layer, which is obtained by performing thermal process into the resultant structure where the floating gate pattern 220 a and the lower selection gate pattern 220 b are formed. As illustrated in FIG. 12c, the first preferred embodiment of the present invention includes island-shaped floating gate patterns 220 a, which are separated from each other on the active regions.

[0047] Referring to FIGS. 13a and 13 b, an upper conductive layer is formed on an entire surface of the resultant structure where the sidewall insulation layer 224 is formed. The upper conductive layer is then patterned to form a control gate electrode 226 a and an upper selection gate 226 b. The control gate electrode 226 a crosses the active region over the floating gate pattern 220 a, and the upper selection gate 226 crosses the active region over the lower selection gate pattern 220 b. The upper conductive layer is preferably a polysilicon layer or a metal polycide layer. In this case, a width of the control gate electrode 226 a is smaller than that of the floating gate pattern 220 a, and a width of the upper selection gate 226 b is smaller than that of the lower selection gate pattern 220 b.

[0048] According to the present invention, the floating gate pattern 220 a conformally covers the first base pattern 204 a. Thus, because an elevation of the floating gate pattern 220 a over the base pattern 204 a is higher than that over the channel region, the floating gate pattern 220 a has a flexural surface. As a result, the nonvolatile memory device of the present invention may have a gate interlayer dielectric layer occupying a large surface area within a limited area.

[0049] Before forming the upper conductive layer, a portion of the interlayer dielectric pattern 222 b is removed from the lower selection gate pattern 220 b. This enables an electrical connection between the upper selection gate 226 b and the lower selection gate pattern 220 b.

[0050] Referring to FIG. 14, impurities may be doped into the source region 208 between the opposite floating gate patterns 220 a, thereby forming a heavily doped source region 230. Likewise, by doping impurities into the drain region 212 between the opposite lower selection gate patterns 220 b, a heavily doped drain region 228 may be formed.

[0051] In another method, referring to FIG. 15, the heavily doped source and drain regions 230 and 228 are not formed. An insulation layer 232 is formed on an entire surface of the resultant structure where the control gate electrode 226 a and the upper selection gate 226 b are formed. The insulation layer 232 is then patterned to form bit line contact holes 234 exposing the drain regions 210. By using the insulation layer 232 as an ion implantation mask, impurities may be implanted into the drain region exposed to the bit line contact holes 234 so that a heavily doped drain region 228 a can be formed. In this case, although not shown in the drawings, contact holes may be formed to expose the source region 208, the control gate electrode 226 a, and the upper selection gate 226 b, respectively, at a predetermined region of the semiconductor substrate.

[0052]FIG. 16 is a top plan view of a nonvolatile memory device according to a second preferred embodiment of the present invention. FIG. 17 is a perspective view of the nonvolatile memory device according to the second preferred embodiment of the present invention.

[0053] Referring to FIGS. 16 and 17, the nonvolatile memory device according to the second embodiment of the present invention is similar to that of the foregoing first preferred embodiment. A difference from the foregoing first preferred embodiment is that a floating gate pattern 320 a and a lower selection gate pattern 320 b have sidewalls aligned to sidewalls of a control gate electrode 326 a and an upper selection gate pattern 326 b, respectively. Thus, in a cross-sectional view of the control gate electrode 326 a, a width of the floating gate pattern 320 a is the same as that of the control gate electrode 326 a, and a width of the lower selection gate pattern 320 b is the same as that of the upper selection gate pattern 326 b.

[0054] Specifically, a device isolation layer is disposed at a semiconductor substrate to define an active region. A memory gate pattern and a selection transistor pattern cross in a row over the active region. The memory gate comprises a control gate electrode 326 a crossing over the active region, and a floating gate pattern 320 a interposed between the control gate electrode 326 a and the active region. The memory gate further comprises a gate interlayer dielectric layer 322 a between the floating gate pattern 320 a and the control gate pattern 326 a. A first base pattern 204 a is interposed between the floating gate pattern 320 a and the active region.

[0055] A channel region exists in the active region between the first and second base patterns 204 a and 204 b. Also, a source region 208, which is separated from the channel region 210, is formed in the active region adjacent to the first base pattern 204 a, while a drain region 212, which is separated from the channel region 210, is formed in the active region adjacent to the second base pattern 204 b. The adjacent source regions 208, which are separated by the device isolation layer FOX, are electrically connected to each other. Preferably, the device isolation layer Fox between the source regions 208 is cut such that the active regions are connected to each other, and the source region 208 is formed in the connected active region.

[0056]FIGS. 18a, 19 a, and 20 are cross-sectional views for illustrating a method of fabricating the nonvolatile memory device according to the second preferred embodiment of the present invention, the cross-sectional views taken along a line III-III′ of FIG. 16.

[0057]FIGS. 18b and 19 b are cross-sectional views for illustrating a method of fabricating the nonvolatile memory device according to the second preferred embodiment of the present invention, the cross-sectional views taken along a line IV-IV′ of FIG. 16.

[0058] Referring to FIGS. 18a and 18 b, first and second base patterns 204 a and 204 b, a lower conductive layer 320, and a dielectric layer 322 may be formed according to the process steps of the first preferred embodiment, as illustrated in FIGS. 7a, 8 a, 9 a, 10 a, 11 a, 7 b, 8 b, 9 b, 10 b and 11B. The dielectric layer 322 and the lower conductive layer 320 are successively patterned to form isolated grooves G on the device isolation layer Fox. A sidewall insulation layer 324 is formed on inner sidewalls of the grooves G.

[0059]FIG. 18c is a top plan view of the resultant structure where the grooves are formed. Referring to FIG. 18c, the grooves G are formed on the device isolation layer Fox between the first base patterns 204 a as well as on the device isolation layer Fox between the tunnel insulation layers 218. In a cross-sectional view of the second base pattern 204 b, widths of the grooves G are preferably designed to be larger than a width of a control gate electrode (326 a of FIG. 17), which will be formed during a subsequent process.

[0060] Referring to FIGS. 19a and 19 b, an upper conductive layer is formed on an entire surface of the resultant structure where the sidewall insulation layer 324 is formed. The upper conductive layer 326 and the lower conductive layer 320 are electrically insulated by the dielectric layer 322 and the sidewall insulation layer 324.

[0061] In another method, although not shown in the drawings, the dielectric layer is formed after forming the grooves. In this case, the lower and upper conductive layers may be insulated only by the dielectric layer.

[0062] Referring to FIG. 20, the upper conductive layer 326, the dielectric layer 322, and the lower conductive layer 320 are successively patterned to form a control gate electrode 326 a crossing the active region, and an upper selection gate 326 b crossing the active region in parallel with the control gate electrode 326 a. A floating gate pattern 320 a is formed between the control gate electrode 326 a and the active region. A gate interlayer dielectric layer 322 a is interposed between the floating gate pattern 320 a and the control gate electrode 326 a. Also, a lower selection gate pattern 320 b, which has sidewalls aligned to sidewalls of the upper selection gate 326 b, is formed under the upper selection gate 326 b. An interlayer dielectric pattern 322 b is interposed between the upper selection gate pattern 326 b and the lower selection gate pattern 320 b. At this time, before forming the upper conductive layer 326, a portion of the dielectric layer 322 is removed such that the upper selection gate pattern 326 b and the lower selection gate pattern 320 b are electrically connected to each other at a predetermined region of the semiconductor substrate.

[0063] The lower selection gate pattern 320 b and the second base patterns 204 b constitute a lower selection gate, and the first base pattern 204 a and the floating gate pattern 320 a constitute a floating gate.

[0064] In a cross-sectional view of the control gate electrode 326 a, the control gate electrode 326 a is patterned such that a width of the control gate electrode 326 a is smaller than widths of the grooves G. As a result, the floating gate patterns 320 a, which are formed to be self-aligned to the control gate electrode 326 a, can be island-shaped.

[0065] Like the foregoing first preferred embodiment, a heavily doped source region 230 may be formed in the source region 208 between the floating gate patterns 320 a, and a heavily doped drain region 228 may be formed in the drain region 212 between the lower selection gates.

[0066] According to the present invention as described above, base patterns are formed before forming a memory gate and a selection gate, thereby forming a source region, a drain region, and a channel region, which are aligned to sidewalls of the base patterns. This makes it possible to prevent changes of a distance from the source region to the channel region due to misalignment of the gates. As a result, it is capable of fabricating a nonvolatile memory device having a cell array in which memory cells, which are formed repeatedly and symmetrical with respect to each other, have uniform threshold voltages.

[0067] While this invention has been particularly shown and described with reference to preferred embodiments thereof, it will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the spirit and scope of the invention as defined by the appended claims. 

What is claimed is:
 1. A method of fabricating a nonvolatile memory device, comprising: forming first and second base patterns on a semiconductor substrate to be separated from each other by a predetermined space; forming a channel region in the semiconductor substrate between the first and second base patterns, and source and drain regions in the active regions adjacent to the first and second base patterns, respectively, wherein the source and drain regions are separated from the channel region by the first and second base patterns, respectively; forming a tunnel oxide layer on a predetermined region of the channel region; forming a memory gate covering the first base pattern and the tunnel oxide layer; and forming a selection gate covering the second base pattern.
 2. The method as claimed in claim 1, wherein forming the first and second base patterns comprises: sequentially forming a gate insulation layer, a base conductive layer and an upper insulation layer on the semiconductor substrate; and successively patterning the upper insulation layer and the first conductive layer, wherein an upper insulation pattern is formed on each of the first and second base patterns.
 3. The method as claimed in claim 2, wherein forming the tunnel insulation layer comprises: forming a first sidewall spacer covering a sidewall of the first base pattern and a sidewall of the upper insulation pattern and at the same time forming a second sidewall spacer covering a sidewall of the second base pattern and the sidewall of the upper insulation pattern; removing a portion of the gate insulation layer adjacent to the first sidewall spacer between the first and second base patterns to expose the semiconductor substrate; forming a tunnel insulation layer, which is thinner than the gate insulation layer, on the exposed semiconductor substrate; removing the upper insulation pattern, and the first and second sidewall spacers.
 4. The method as claimed in claim 3, wherein the upper insulation pattern, and the first and second sidewall spacers are composed of an insulation layer having an etch selectivity with respect to the gate insulation layer and the tunnel insulation layer.
 5. The method as claimed in claim 1, wherein forming the memory gate comprises: forming a floating gate pattern covering the first base pattern and the tunnel oxide layer; forming a gate interlayer dielectric layer on the floating gate pattern; and forming a control gate electrode on the gate interlayer dielectric layer.
 6. The method as claimed in claim 5, wherein forming the floating gate and the gate interlayer dielectric layer comprises: forming a lower conductive layer conformally covering the first base pattern and the channel region; forming a dielectric layer covering the lower conductive layer; successively patterning the dielectric layer and the lower conductive layer to form a floating gate pattern and a gate interlayer dielectric layer, which are sequentially stacked; and forming a sidewall insulation layer covering a sidewall of the floating gate pattern.
 7. The method as claimed in claim 5, wherein forming the floating gate pattern and the gate interlayer dielectric layer comprises: forming a lower conductive layer conformally covering the first base pattern and the channel region; patterning the lower conductive layer to form a floating gate pattern on the first base pattern and the tunnel oxide layer; and conformally forming a dielectric layer on the floating gate pattern.
 8. The method as claimed in claim 5, wherein forming the floating gate pattern, the gate interlayer dielectric layer and the control gate electrode comprises: sequentially forming a lower conductive layer, a dielectric layer, and an upper conductive layer that conformally cover the first base pattern and the channel region; and successively patterning the upper conductive layer, the dielectric layer, and the lower conductive layer.
 9. A method of fabricating a nonvolatile memory device, comprising: forming a device isolation layer at a predetermined region of a semiconductor substrate to define an active region; forming a gate insulation layer on the active region; forming first and second base pattern crossing over the semiconductor substrate, the first and second base patterns being separated from each other by a predetermined space; forming a channel region in the active region between the first and second base patterns, and source and drain regions in the active regions adjacent to the first and second base patterns, respectively, wherein the source and drain regions are separated from the channel region by the first and second base patterns, respectively; etching a portion of the gate insulation layer between the first and second base patterns to expose a predetermined region of the semiconductor substrate; forming a tunnel insulation layer on the exposed semiconductor substrate; and forming a memory gate and a selection gate crossing the active region in a row, wherein the memory gate includes a floating gate pattern, a control gate electrode, and a gate interlayer dielectric layer, wherein the floating gate pattern covers the first base pattern and the tunnel insulation layer, and an edge of the floating gate pattern overlaps with the device isolation layer, and wherein the control gate electrode crosses the active region over the floating gate pattern, and the gate interlayer dielectric layer is interposed between the floating gate pattern and the control gate electrode.
 10. The method as claimed in claim 9, wherein forming the first and second base patterns comprises: sequentially forming a base conductive layer and an upper insulation layer on the gate insulation layer; and successively patterning the upper insulation layer and the base conductive layer to form the first and second base patterns, and an upper insulation pattern covering the first and second base patterns, wherein the upper insulation pattern is removed after forming the tunnel insulation layer.
 11. The method as claimed in claim 10, wherein forming the tunnel insulation layer comprises: forming first and second sidewall spacers covering sidewalls of the first and second base patterns, respectively; forming a photoresist pattern exposing a portion of the gate insulation layer between the first and second base patterns; etching the gate insulation layer by using the photoresist pattern as an etch mask to expose a predetermined region of the semiconductor substrate; removing the photoresist pattern; forming a tunnel insulation layer on the exposed semiconductor substrate; and removing the upper insulation pattern, and the first and second sidewall spacers.
 12. The method as claimed in claim 11, wherein the photoresist pattern is formed to expose the first sidewall spacer between the first and second base patterns, and a portion of the gate insulation layer adjacent to the first sidewall spacer.
 13. The method as claimed in claim 9, wherein forming the memory gate and the selection gate comprises: sequentially conformally forming a lower conductive layer and a dielectric layer on an entire surface of the resultant structure where the first and second base patterns are formed; successively patterning the dielectric layer and the lower conductive layer to form a floating gate pattern and a gate interlayer dielectric layer as well as a lower selection gate pattern and an interlayer dielectric pattern, wherein the floating gate pattern and the gate interlayer dielectric layer are sequentially stacked on the active region to cover the first base pattern and the tunnel insulation layer, and wherein the lower selection gate pattern and the interlayer dielectric pattern are sequentially stacked to be separated from the floating gate pattern by a predetermined space, and cross the active region, wherein the floating gate pattern and the gate interlayer dielectric layer are extended to a top of the device isolation layer such that edges of the floating gate pattern and the gate interlayer dielectric layer overlap with the device isolation layer; forming a sidewall insulation layer on sidewalls of the floating gate pattern; forming an upper conductive layer on an entire surface of the semiconductor substrate where the sidewall insulation layer is formed; patterning the upper conductive layer to form a control gate electrode and an upper selection gate, wherein the control gate electrode crosses the active region over the gate interlayer dielectric layer, and the upper selection gate crosses the active region in parallel with the lower selection gate pattern over the interlayer dielectric pattern.
 14. The method as claimed in claim 9, wherein forming the memory gate and the selection gate comprises: sequentially conformally forming a lower conductive layer and a dielectric layer on an entire surface of the resultant structure where the first and second base patterns are formed; successively patterning the dielectric layer and the lower conductive layer to form grooves that expose a portion of the device isolation layer, wherein the grooves are formed in parallel with the active region on the device isolation layer of both sides of the active region; forming a sidewall insulation layer on inner sidewalls of the grooves; forming an upper conductive layer on an entire surface of the resultant structure where the sidewall insulation layer is formed; and successively patterning the upper conductive layer, the dielectric layer, and the lower conductive layer, to form a memory gate pattern and a selection gate pattern that cross the active region in a row, wherein the memory gate includes a control gate electrode crossing the active region, a floating gate pattern disposed under the control gate electrode, and a gate interlayer dielectric layer interposed between the floating gate pattern and the control gate electrode, wherein the floating gate pattern covers the first base pattern and the tunnel insulation layer, and have a sidewall parallel with the active region on the device isolation layer, and wherein the selection gate pattern includes a lower selection gate pattern, an interlayer dielectric pattern, and an upper selection gate pattern, which are sequentially stacked and cross the active region. 